In modern commercial passenger aircraft, a problem that is being encountered is interior cabin noise caused by engine vibrations transmitted through the airframe structure. For example, vibrations of a wing-mounted engine may be transmitted by the engine mounts, strut, wing, and fuselage. The problem of cabin noise is particularly troublesome in some of the more recent aircraft which have propulsion systems closely coupled to the wing through a very stiff mount, nacelle, and strut structure.
Some engine installations react roll torque at the front mount. This arrangement permits vertical and lateral vibrations to be isolated independently at the rear mount without affecting the roll stiffness of the mount installation. However, in wing mounted engines it is generally advantageous to react roll torque at the rear mount rather than at the front mount. Modern engines possess high torsional stiffness and strength, and when roll torque is reacted at the rear mount, the engine rather than the strut will transmit torque rearwards. This reduces the load carrying requirements of the strut and thereby permits weight savings in the strut. In addition, rear mount torque reaction will generally permit the strut width to be narrower to decrease drag and enhance the aerodynamic performance of the aircraft. A problem that is encountered in providing vibration isolation in a rear mount that reacts roll torque is the need to maintain rigidity in the lateral and roll directions in order to prevent redistribution of lateral loads and roll torque to the front mount and thrust reverser hinges. Known isolation systems generally reduce the torsional stiffness of the rear mount and thereby permit such undesirable redistribution of loads.
There are a number of approaches which may be taken to solving the problem of interior cabin noise caused by engine vibrations. One such approach would be to reduce engine unbalance levels. Since the problem of cabin noise can occur even though the engines have normal levels of unbalance within engine specification limits, the approach of reducing engine unbalance levels would be very time consuming and expensive to carry out. To be effective, the engine balancing would have to be done with the engine installed on the wing because vibration response is different in an installed engine than on an engine test stand. Normal levels of rotor unbalance are very small and are caused by a number of factors that vary randomly. Particular combinations of these variables will cause an unacceptable level of cabin noise. The readjustment of the variables that are related to cabin noise is a trial and error process which would be very expensive for new engines and highly, if not completely, impractical for engines being overhauled in maintenance shops.
Another possible approach would be to use tuned mass vibration absorbers to reduce cabin noise. Mass vibration absorbers have been used successfully in limited situations in which the vibrations to be reduced were over a very narrow frequency range. However, a number of problems arise in the use of mass vibration absorbers. These problems include the tendency of the absorbers to drift from their tuned frequencies, fatigue failures of the absorbers, and the relatively high weight of absorbers that reduce vibrations at low frequency levels in the range of about 0 to about 60 hertz.
A third and more conventional approach to solving the problem of cabin noise is to provide vibration isolators at the engine mounts. Problems that are associated with the use of known engine mount isolators include the undesired reduction in torsional stiffness and redistribution of side loads and torque discussed above, space limitations in the engine mount area, and limitations on the isolator configuration and/or material because of the high temperature environment in the engine mount area. The use of compressed woven wire isolation material instead of elastomeric materials has been proposed for use in high temperature environments. The proposed isolator designs using such metal materials have a number of drawbacks. The designs generally are unacceptable for installations in which roll torque is reacted at the rear mount because they reduce the rear mount lateral load and torsional stiffness to an unacceptable degree and thereby permit excessive redistribution of side loads and roll torque into the forward mount and thrust reverser hinges. Metal mesh isolators also provide relatively unpredictable isolation because of their high degree of nonlinear behavior. In addition, the ability of such isolators to provide sufficient damping to function adequately in a system resonant mode and to isolate the low frequency vibrations associated with cabin rumble noise is questionable.
The patent literature includes numerous examples of aircraft engine mount systems. Wing engine mount systems without vibration isolation are disclosed in U.S. Pat. No. 3,318,554, granted May 9, 1967, to P. A. Ward et al.; No. 3,844,115, granted Oct. 29, 1974, to W. B. Freid; and No. 4,013,246, granted Mar. 22, 1977, to D. J. Nightingale. Aircraft engine mount systems with vibration isolation are disclosed in U.S. Pat. No. 1,815,442, granted July 21, 1931, to A. F. Masury; No. 1,860,444, granted May 31, 1932, to L. M. Woolson; No. 2,523,504, granted Sept. 26, 1950, to F. A. Ford, Jr.; No. 2,715,508, granted Aug. 16, 1955, to L. C. Small, Jr.; No. 2,722,391, granted Nov. 1, 1955, to R. T. Krieghoff; No. 2,724,948, granted Nov. 29, 1955, to G. H. Hiscock et al.; No. 3,288,404, granted Nov. 29, 1966, to W. E. Schmidt et al.; No. 3,836,100, graned Sept. 17, 1974, to P. W. Von Hardenberg et al.; No. 4,097,011, granted June 27, 1978, to R. F. White; and No. 4,111,386, granted Sept. 5, 1978, to I. J. Kenigsberg et al. Krieghoff, Hiscock et al., Schmidt et al., and Kenigsberg et al. disclose isolators that have stacks of alternating metal plates and elastomeric layers. Von Hardenberg et al. disclose an engine mount for a helicopter that is designed to isolate the engine from lateral input motions imparted by the airframe by uncoupling engine roll response. There are separate laterally offset upper and lower rear mounts. The isolator in the lower mount provides a minimum lateral restraint and elastic vertical restraint of the engine, and the isolator of the upper mount provides a minimum vertical restraint and elastic lateral restraint of the engine.
U.S. Pat. No. 2,028,549, granted Jan. 21, 1936, to H. C. Lord, discloses an automobile engine mounting system in which the engine is suspended from front and rear U-shaped frames. At each U-shaped frame, two links connect the engine to the corners of the frame. The joints between the links and the frame members and the joints between the links and the engine have elastomeric bushings to absorb vibrations and torque impulses. The links are oriented at about 45 degrees and swing to accommodate rocking of the engine due to engine torque impulse. The rubber bushings around the joints distort to accommodate the motion.
The above patents and the prior art that is discussed and/or cited therein should be studied for the purpose of putting the present invention into proper perspective relative to the prior art.